1. Field of the Invention
The present invention relates to a holder furnace for containing a supply of molten metal and, more particularly, to a pressurized and bottom heated holder furnace for containing a supply of molten metal.
2. Description of the Prior Art
Molten metal holding furnaces, or holder furnaces, are used in the art for holding and/or melting molten metal. Holding furnaces are often used to contain a supply of molten metal for injection into a casting machine. For example, U.S. Pat. No. 4,753,283 to Nakano discloses a horizontal injection molten metal is maintained in a holding furnace which periodically provides molten metal to the casting machine. Molten metal from a larger smelting furnace is supplied periodically to the holding furnace to maintain a set amount of molten metal in the holding furnace. The holding furnace is heated by a burner located adjacent a sidewall of the holding furnace.
In addition to the burner arrangement disclosed by the Nakano patent, several other methods are known in the art for heating molten metal contained in a holding furnace. Several common methods include induction heating, radiant heating, and immersion heating. For example, U.S. Pat. No. 4,299,268 to Lavanchy et al. discloses a molten metal casting arrangement in which molten metal is contained in a large capacity pressure ladle (i.e., holding furnace) that is heated by a heating inductor located at the bottom of the pressure ladle. The pressure ladle periodically supplies molten metal to a smaller capacity tilting ladle, which supplies molten metal to a casting apparatus. U.S. Pat. No. 3,991,263 to Folgero et al. discloses a similar molten metal holding system to that disclosed by the Lavanchy et al. patent, but the system disclosed by the Folgero et al. patent is pressurized.
U.S. Pat. No. 4,967,827 to Campbell discloses a melting and casting apparatus in which electric radiant heating elements are used to heat molten metal passing from a holding furnace to a casting vessel. U.S. Pat. No. 5,398,750 to Crepeau et al. discloses a molten metal supply vessel in which a plurality of electric immersion heaters is used to heat molten metal in a holding furnace. The immersion heaters extend downward from the holding furnace cover and are partially submerged in the molten metal contained in the holding furnace. U.S. Pat. No. 5,567,378 to Mochizuki et al. discloses a similar immersion heater arrangement to that found in the Crepeau et al. patent.
The above-discussed radiant heating and immersion heating elements for heating molten metal in a holding furnace are located above the surface of the molten metal and are xe2x80x9ctopxe2x80x9d heating arrangements. The xe2x80x9ctopxe2x80x9d heating arrangements known in the art require a significant amount of space above the holding furnace for the individual heating elements. For example, the immersion heaters and electric radiant heaters discussed previously in connection with the Crepeau et al. and Campbell patents require a significant amount of space above the surface of the molten metal in the holding furnace, as well as a support structure above the holding furnace for supporting the heating elements above the surface of the molten metal. External heating arrangements, such as the burner arrangement disclosed by the Nakano patent, heat the holding furnace along a bottom wall or sidewall of the holding furnace, and typically require space along the sides or bottom of the holding furnace for the heating elements. With such top/external heating arrangements, it is difficult to maintain a constant molten metal temperature in the holding furnace.
An alternative to top/external heating arrangements is to provide bottom heating devices in holding furnaces. Such bottom heating devices are typically embedded within the bottom wall of the holding furnace. One known bottom heating arrangement in a molten metal holding furnace is disclosed by U.S. Pat. No. 5,411,240 to Rapp et al. The heating cycle of such bottom heating arrangements places significant stress on the bottom wall of the holding furnace. Such bottom heating arrangements are also generally unsuitable for use with containment difficult metals such as molten aluminum alloys. Any leakage of molten aluminum alloy into the bottom wall of the holding furnace will cause failure of the heating elements.
In view of the foregoing, an object of the present invention is to provide a bottom heated holder furnace having improved molten metal containment characteristics. In addition, it is an object of the present invention to provide a bottom heated holder furnace that is suitable for use with molten aluminum alloys. It is a further object of the present invention to provide a holder furnace that may be cyclically pressurized without large pressure drops occurring within the holder furnace.
The above objects are accomplished with a pressurized molten metal holder furnace in accordance with the present invention. The holder furnace includes a storage vessel having sidewalls and a bottom wall defining a molten metal receiving chamber for containing the supply of molten metal. At least one furnace insulating layer lines the molten metal receiving chamber of the storage vessel. A thermally conducted heat exchanger block is located at the bottom of the molten metal receiving chamber for heating the supply of molten metal. The heat exchanger block has a top face, a bottom face, and side faces. The heat exchanger block includes a plurality of electrical heaters extending therein and projecting outward from at least one of the faces of the heat exchanger block, and further extending through the furnace insulating layer and at least one of the sidewalls of the storage vessel for connection to a source of electrical power. A sealing layer at least partially covers the bottom face and side faces of the heat exchanger block such that the heat exchanger block is substantially separated from contact with the furnace insulating layer. A gas pressurization valve is in fluid communication with the molten metal receiving chamber and the interior of the heat exchanger block through the electrical heaters. The gas pressurization valve is configured for connection to a gas pressurization source, and further configured to pressurize the molten metal receiving chamber and the heat exchanger block upon connection to the gas pressurization source and activation of the gas pressurization valve.
The holder furnace may include a cover positioned on top of the storage vessel and enclosing the molten metal receiving chamber. The cover may include a first conduit extending therethrough and in fluid communication with the gas pressurization valve for pressurizing the molten metal receiving chamber. The cover may further include a second conduit extending therethrough for removing molten metal from the molten metal receiving chamber upon pressurization.
The portion of the electrical heaters extending outward from the sidewall of the storage vessel may be enclosed in a chamber connected to the gas pressurization valve and configured for pressurization upon activation of the gas pressurization valve. The sealing layer may be an alumina fiber mat. The heat exchanger block may be made of graphite, silicone carbide, or another substantially equivalent material.
The electrical heaters may extend between opposite sidewalls of the storage vessel and through the heat exchanger block. The electrical heaters may each include a continuous heating element extending through at least one of the opposite sidewalls, the at least one furnace insulating layer, and extending at least partially through the heat exchanger block. The electrical heaters may each further include respective tubes extending through the opposite sidewalls, the at least one furnace insulating layer, and extending at least partially into opposite faces of the heat exchanger block. The heating element for the electrical heaters may extend at least partially through each of the respective tubes. Sealing gaskets may be positioned within the heat exchanger block. The sealing gaskets may cooperate, respectively, with ends of the tubes extending into the opposite faces of the heat exchanger block for preventing molten metal from leaking into the tubes and contacting the heating element of the electrical heaters. The tubes may be ceramic insulating tubes that are substantially surrounded by a layer of ceramic fiber rope for preventing molten metal from the supply of molten metal from leaking into the ceramic insulating tubes and contacting the heating elements of the electrical heaters.
Flange plates may be attached, respectively, to the ceramic insulating tubes at the opposite sidewalls of the storage vessel. The ceramic insulating tubes may be held into compression against the opposite sidewalls of the storage vessel via the flange plates, bolts, and a plurality of Belleville washers act to yield about 170 pounds of torque on each of the ceramic insulating tubes.
The sealing layer may further extend along a portion of the top face of the heat exchanger block. The furnace insulating layer may overlap the sealing layer extending along the top face of the heat exchanger block. The portion of the top face of the heat exchanger block having the sealing layer thereon may define a non-linear path such that any molten metal leakage into the furnace insulating layer follows a torturous path along the sealing layer. A portion of the top face of the heat exchanger block having the sealing layer thereon may also define a plurality of ribs such that any molten metal leakage into the furnace insulating layer follows a torturous path along the sealing layer.
Further details and advantages of the present invention will become apparent from the following detailed description in conjunction with the drawings wherein like parts are designated with like reference numerals throughout.